The present invention relates to gas turbine engines, especially to a gas turbine combustion system, and more particularly to a cyclone combustor which has premixed fuel/air mixture tangentially injected into the combustor.
Industrial gas turbine engines must operate under increasingly stringent emissions requirements. In order to have a marketable power generation product, an engine producing the lowest possible emissions is crucial. Emissions of nitrogen oxides NOx and carbon monoxide (CO) must be minimized over specified engine operating ranges. To achieve this low level of emissions the combustion system requires the complete burning of fuel and air at low temperatures.
The current technologies for achieving lower NOx may require fuel and air to be premixed before entering the combustor. Combustors that achieve lower NOx emissions without water injection are known as dry-low-emissions (DLE) and offer the prospect of clean emissions combined with high engine efficiency. This technology relies on a high air content in the fuel/air mixture.
In a DLE system, fuel and air are lean-premixed prior to injection into the combustor. However, two problems have been observed. The first is combustion instability or unstable engine operability which results in noise, and the second is the related CO emissions. The stability of the combustion process rapidly decreases at lean conditions and the combustor may be operating close to its blow-out limit because of the exponential temperature dependence of the chemical reactions. This can also lead to local combustion instabilities which change the dynamic behaviour of the combustion process, and endanger the chemical integrity of the entire gas turbine engine. This is because several constraints are imposed on the homogeneity of the fuel/air mixture since leaner than average pockets of mixture may lead to combustion stability problems, and richer than average pockets will lead to unacceptably high NOx emissions. At the same time, a substantial increase in CO and unburned hydrocarbon (UHC) emissions as a tracer for combustion efficiency is observed, which is due to the exponential decrease in chemical reaction kinetics at leaner mixtures for a given combustor. Therefore, efforts have been made in development of novel fuel mixing and burning devices.
It is well known that in general, injection of fuel/air mixtures tangentially into the combustor will provide optimum circulation of fuel/air mixture in the combustor to improve combustor life span and flame stability. An example of a cyclone or vortex type combustion chamber is described in U.S. Pat. No. 2,797,549 to Probert et al. on Jul. 2, 1957. The cyclone or vortex type combustion chamber described by Probert et al. includes three fuel premixing chambers tangentially oriented with respect to the combustion chamber. Incoming air is directed into the tangential premixing chambers and is mixed with the fuel supply therein before being injected into the helical vortex of the combustion chamber.
Nevertheless, the fuel/air mixture is generally flammable so that undesirable flashback into the premixer section is possible. Furthermore, gas turbine combustors utilizing lean premixed combustion typically require some conversion from a premixed to a non-premixed (diffusion) operation at turn-down conditions, to maintain a stable flame. Such conversion capability introduces undesirable design complexities and generally raise costs. The disadvantages of premixing have been recognized in the industry and therefore there is a need for new combustion systems using a premixed fuel/air mixture to overcome these problems.
One object of the present invention is to provide a cyclone combustor for a gas turbine engine which provides an optimized circulation of a premixed fuel/air mixture in the combustor.
Another object of the present invention is to provide a combustor using a premixed fuel/air mixture while inhibiting undesirable flashback into the premixer section.
In accordance with one aspect of the present invention, a combustor is provided for a gas turbine engine which comprises a substantial cylindrical combustor can and a plurality of fuel and air premixing tubes. The combustor can has a central axis and includes an upstream end wall and a continuous side wall around the central axis thereof for receiving fuel and air to produce combustion products for the engine. The respective premixing tubes are attached to the side wall of the combustor can and are in fluid communication with the combustor can. The premixing tubes are positioned adjacent to the upstream end wall and are circumferentially spaced apart from one another. Each of the premixing tubes includes a major tube section for producing a fuel/air mixture therein and an outlet section for injecting the fuel/air mixture into the combustor can for combustion. The major tube section has a central axis thereof parallel to the central axis of the combustor can, and the outlet tube section has a central axis thereof extending substantially perpendicularly to the central axis of the major tube section and being oriented toward the combustor can radially, with a tangential offset.
The tangential offset of each premixing tube with respect to the combustor can is determined with a parameter T, preferably {fraction (1/24)}T less than T less than ⅙D wherein D is the length of a diameter of the combustor can and T is the distance between the central outlet section axis of the premixing tube and a diametrical line of the combustor can, the diametrical line being parallel to the central outlet section axis. It is preferable that at least one of the premixing tubes is adapted to be individually staged, producing the fuel/air mixture with a selected mixing ratio, or delivering pure air.
The cyclone combustor of the present invention uses a novel premixer scheme to optimize performance. The tangential offset of the premixing tubes is designed to provide an optimized circulation in the combustor can for liner life span, flame stability and engine turn-down operation which requires a minimum flameout fuel/air ratio, as well as for low combustion noise and low emission levels. The ignition and pilot fuel system is placed to take advantage of the premixing tube entry locations as well as the direction of mixture flow momentum. Furthermore, the specific combination of parallel axes of the fuel combustor can and the premixing tubes provides a right angle between the outlet section and the major tube section of each premixing tube such that flashback into the premixing tube is effectively inhibited.
The cyclone combustor of the present invention is able to meet the current requirements for emissions, i.e. NOx emissions lower than 10 ppm and CO emissions lower than 10 ppm.
Other advantages and features of the present invention will be better understood with reference to a preferred embodiment of the present invention described hereinafter.